Pressure measuring apparatus



Oct. 28, 1941. H. P. KUEHNI PRESSURE MEASURING APPARATUS Filed July 26, 1940 llllll? WEEEEE Hlnveimzowg ans. U6 I,

His Attorney Patented Oct. 28, 1941 I 2, .v

UNITED STATES QPATENT. OFFICE PRESSURE MEASURING XPPABATUS Hans P. Kuelmi, Schenectady, N. Y.,- assignor to General Electric Company, a corporation of New York Application July 26, 1940, Serial No. 347,711

' 2 Claims. (Cl. 177-351) This invention relates'to electrical apparatus in-partial section of apparatus constructed in for the measurement of pressures and more paraccordance with my invention, and Fig. 2 is a ticularly to improved apparatus for measuring diagram of connections which shows a manner. steady-state or transient fluid or gas pressures of connecting a portion of the apparatus of 13.) by electromagnetic means. 5 in an electrical circuit for carrying out my inven- It is an object of my invention to provide a new tion. l r I I and improved pressure measuring device adapt- Referring to Fig. l, I have shown a\pressure ed for the measurement of gaseous or liquid pre's-' S ge c nstructed in accordan with my invensures of either steady or rapidly varying value. tion in which a body member I0 is provided with It is another object of my invention to provide th d p n I w h is adapted tovbe sea new and improved pressure gage in which temcured into the wall of a chamber I wherein perature effects of the liquid or gas whose pres-H measurements of p r r d r T o y sure is to be measured are substantially elimimember "I is further provided with a central nated from the measurement results. n s'having an upper p rt n 6 of reduced It is a further object of my invention to provide diameter d a owe portion l8 forming a meman improved pressure measuring device which is her or recess of larger diameter which is' subof relatively simple o t ti and which may stantially co-extensive with the threaded portion be built for comparatively'lowoosL 5 l2. Positioned within the lower chamber or re- Other objects and advantages. of my invention o ss "I n co w th the. p e su e will become apparent as the description proceeds. go chamber I0 is a hi hly flexible member in In carrying out my invention in it preferred the form of a bellows comprising flexible or 06rform I provide gagehead having a, highly flexrugated side walls 22 and 8. flat end portion 2 ible member which is positioned in a manner to to Which the force due to t P e re in the seal an opening communicating with a chamber chamber 14 is pp T end P r ion lies in in which the pressure is to be measured. A pres- 25 e Pl ne substantially normal to the direction of sure sensitive member in the form of an elastic movement a d conveniently may be r ed to or resilient member h as a diaphragm or b as the active pressure surface. The expansible is positioned in such a way as to oppose the force m mber 20 is provided at its upp r portion with exerted on the highly flexible member and thus n a n lar flange 26 which is secured in Sealed under observation.

restrain the motion of the latter member. The 30 leletion'to a Shoulder f the dy em er highly flexible member is of negligible stiffness by Suitable means such -8 Weldin or sold rin8- in the axial direction compared with the resn- The flexible member 20 y e o st ucted of lent member so that any unequal expansion in a suitable ateria u f r p as Ph sthe gage parts is not transmitted as a force on the phor-bienze o nickel dep n n on the p resilient member. In the preferred arrangement, atures encountered and the tendency of the liqthe armature element of an electromagnetic gage uid or gas to cause corrosion. I wish to point out is mounted in such a way that it is moved in acthat while BV hown the end portion 24 to cordance with the deflection or deformation of w i the act ve P e s e is applied as being P- the pressure responsive member, 1 t advanarate from the flexible or corrugated portion", tage' of the fact that variations in the'air gap of 40 these W elem n s may be originally made as a magnetic circuit cause a change in the reluc-' P member s ead of being made separate and tance of the circuit and by changing this air gap integrally t d y Welding 0 t e like. So'far in response to and in proportion to the pressures as the characteristics of the fiexible member 20 under observation I am able to influence suitable are concerned, it should be highly flexible in the instruments to indicate or record the pressures directi of s axial movement nd l ively stifl in a direction transverse to thismovement. The novel features which I believe to be char- I wish to point out in this connection that inacteristic of my invention are set forth with parstead of employing a flexible member in the form ticularity in the appended claims. My invention of a bellows I may also employ a very thin disk. itself, however, both as to its organization and shaped flexible member in the form of a diamethod of operation together with further obphragm. Jects and advantages thereof may best be under- Positioned in supported relation on the body stood by reference to the following description member Ills a main pressure-responsive resiltaken in connection with the accompanying lent or elastic element 30 here shown as 9. diadrawing in which Fig. 1 is a side elevational view phragm and which is adapted to withstand d being resilient or elastic, returns to its original act. in opposition to the forces-produced on the flexible member 20 by theliquld or gas in the pressurechamber. The diaphragm 30 is provided with a relatively thick outer annular portion 32 and a central portion 34 which are joined '5 to each other by a comparatively thin intermediate portion 36. The annular portion 32 of the diaphragm 30 is rigidly c1amped.between the upper surfacelof the supporting member l and an annulus or disk 38 by means of a plurality of w threaded bolts 40; A longitudinal member or push rod 42 serves to transmit the forces applied to the surface 24 of the flexible member 20 due to pressure variations inthe chamber l4 to the central portion 34 of the diaphragm 20. The

1 upper portion of the 'rod 42 is provided with a threaded portion 44 which passes through a ,central annular opening 46 in the annulus /or--disklike member 28, which threads engage the central or hub-like portion 34v of the diaphragm so. A look nut' 48 serves to hold the push rod 42 firmly in position. To further reduce the transmission .of heat through the push rod 42 to the H main pressure sensitive member" from the active pressure surface 24, I prefer to insert a'suitable heat insulating member 25 between the members .24 and 42 when the push rod is composed of a heat conducting material. However, the push rod itself may be composed of a heat in-f sulating material, in which case the member 25 so may be dispensed with. g I wish to point out that while I have shown the pressure responsive resilient or elastic ele-v ment in the form of a diaphragm it is not my intention to limit the invention in this respect as I may employ a resilient member in the form of a beam supported at both ends or of the cantilever construction, the beam preferably-be- I ing formed integrally in the member 30 in a manher, for example, as described and claimed in my co-pending application Serial No. 318,683, filed February 13, 1940, entitled "Pressure measuring apparatus, and assigned to the assignee of the present application. I

It is pointed out that it is the pressure responsive resilient member 30 which acts to restrain the motion of the push rod 42 due to forces produced on the active pressure surface 24 by the liquid or gas under observation. The member II,

position when the forces are removed. By means 50 of the corrugated portion 22 the member 20 is made stifi in the transverse or diametrical direction but is highly flexible in the direction of axial movement. In other words, it is of such a construction that it has a negligible stiffness in the direction of axial movement as compared tothe member 30. Thus the flexible member 2|) acts as a sealing device without mechanically restraining the deformation of the main diaphragm. .Thus if there are unequal temperature expansions between the push rod 42- and the member Ill due to hot gases or liquids in the chamber l4, the flexible member 20 will allow the differences of expansion to take place without producing a force on the resilient member 30.

In accordance with the illustrated embodiment, I have provided an electromagnetic gage for measuring the deflection, movement, or defor-- mation of the resilient or elastic member 30 due to forces exerted on the flexible member 20 in accordance with pressure variations in the chamber l4. The gage comprises a pair of .substantially E-shapedmagnetic cores 50 and 52 on the central legs of which are. positioned electrical coils or windings l4 and I0, respectively. The E-shaped core I2 may be suitably mounted on the annular member or disk 38 in any convenient manner, and the E-shaped core 50 is positioned with its open ends or legs in opposed"and spaced relation to the open ends or. legs of the core 52. Positioned intermediate the magnetic cores 50 and I2 and in cooperating relation therewith is a movable magnetic armature member 58 in the "form of a cantilever beam which is mounted at its fixed .end by means of a bolt 60 on a stud or supporting member 02 which projects from the disk or annulus 30 In the arrangement illustrated, the beam," is provided with a reduced cross-section or cut-away portion 64, about which deflection of the beam takes place, in order that the armature or beam shall have substantially ture at a point intermediate its support .84 and the gage coils for transmitting to the armature the deflection of the pressure sensitive member 10. A plurality of lock nuts it serve to adjust the position of the armature and to hold the members firmly in position. The magnetic cores 50 and I2 and the armature member 58 are pref erably composed of high permeability magnetic material, and this material may be laminated depending upon the frequency of the electrical circuit in which the arrangement is to be operated.

In Fig. 2, I have shown an electrical circuit diagram of connections which may be conveniently employed formeasuring variations of the reactance of the coils I4 and B6 in accordance with movement of the armature 58 due to pressure variations in the'chamber l4. In'the arrangement illustrated, aasource of alternating current voltage 08 is arianged to energize a Wheatstone bridge circuit which comprises two differentially-connected electrical circuits, one of which comprises a section 60 of a differential 'reactor 10 or a transformer and the gage coil trically similar so that with currents of equal magnitude and of the same phase relation 'flowing in the differential circuits, the voltage between the points H and I2 willbe zero. The frequency of the source of supply 68 to be employed will depend upon the frequency of the pressure variations to be measured as will be understood by those skilled in the art. For example, when steady or slowly varying pressures are to-be measured the source of supply 68 may be an ordinary commercial lighting circuit but for pressures which vary rapidly or for transient pressures a higher frequency supply such as 2000 cycles may be found to give moresatisfactory results.

The conjugate portion of the Wheatstone bridge circuit, as represented by the points II and 12, is shown connected to the input terminals of a full-wave rectifier element 13, theoutput terminals of which may be connected to a direct current indicating instrument -14. If it is desired to record the pressure variations. arecording element such, for example, as an oscillograph 'aaeacaa sive devices 14 and I5. For example, if the bridgels energized from a 2000 cycle source of supply,

there will be a 4000 cycle frequency ripple in the 10 output in the case where a fullwave rectifier is heat transmission through the upper section of the push rod 42 from the diaphragm 30 to the gage armature II and from the diaphragm to the gage coil supports tend to be unequal resulting in used. Thus. I may employ a filter comprising an inductance element 11 and a capacitor II, with the elements tuned to resonate at the 4000 cycle frequency. As will be understood by those skilled in the art. in operation, the filter l0 presents a practically infinite impedance to the flow of currents having a frequency corresponding to its resonating frequency so that such currents will have no effects on the measurement results. Also, due to the imperfection of rectifier-s such as I! there may be objectionable 2000 cycle frequencies present and I may in such case employ an additional filter element 19, similar to the filter 16, with the elements tuned to resonate at $0 cycles. Furthermore, it may be that obiectionable harmonic frequencies are present in the source of supply 68. In such an event, I may un ai expansion of these parts. Thus the arma It would move relative to the cores 50 and II due merely to temperature effects. Consequently, with such an arrangemenhwhen' high 7 temperatures are encountered, it is difiicult to prevent changes of the gage instrument reading due to these temperature efiects.

By the employment of an auxiliary member such as the highly flexible element 20 which en- 15 closes the cooperating end of the push rod 42 and further by providing an insulating member 26 between the rod and bellows, the hot gases or liquids under observation do not act directly on the pressure restraining resilient member or diaphragm 30 but act on the flexible element or bellows 20 instead. Thus the high temperatures are kept away from the diaphragm 30 and from the rest of the upper structure of the gage head. Inasmuch as the main diaphragm 30 is comparatively stiff relative to the highly flexible member 20 the former is practically the sole means of restraint to motion in an axial direction of the active pressure surface 24. Consequently, any unequal or relative expansions between the push rod 42 and the member l0, which feet the measurements results. The foregoin arrangement is described and claimed in my previously-referred-to co-pending application.

The magnitude of the potential of the supply source 68 is adjusted to the desired value and the air gaps between the cores 50 and 52 and the armature 58 are adjusted to give the desired sensitivity. The pressure gage may be calibrated In any well-known manner such, for example, as by connecting the pressure detector to a gage tester or to a reservoir or chamber, the static pressure of which can be measured.

Assuming that it is desired to measure the pressures in a chamber of any sort, the threaded end portion of the device is screwed into a threaded aperture in the chamber containing the gas or liquid the pressure of which is to be measured. The pressure of the liquid or gas in the chamber is in communication with the active pressure surface 24 of the flexible member 20 thereby exerting a force thereon which force is transmitted to the diaphragm 30. The resulting deformation or movement of the. resilient member 30 causes a deflection of the magnetic armature 58. Increasing pressures produce an increase in the air gaps formed between the magnetic core I: and the armature 58 and a decrease in the air gaps formed between the magnetic core 50 supports the main diaphragm and the flexible member 20, are allowed to take place without producing a force on the pressure responsive diaephragm 30, such unequal expansion being taken up by movement of the flexible member 20. As

a result of the foregoing arrangement temperature effects are practically eliminated.

The arrangement has another distinct advantage in the case where carbon deposits or other 40 floating particles are present in the liquid or gas the pressure of which is being investigated. In the case where piston type gages are employed, such particles often wedge themselves between the cylinder and the piston wall thereby tending to cause a sticking of the gage. Furthermore, in

certain applications where corrosive gages or liquids are being investigated the cylinder and piston would tend to deteriorate. The same holds true in the case where the gases or' liquids are in direct contact with the main diaphragm or pressure responsive element. The present arrangement largely overcomes these dimculties because of the fact that the flexible element 20 is sealed against the shoulder 28 of the member l0 and due to the simplicity of the member 20 it may and the armature 58. As a result, the reactance of one of the gage coils is increased and that of the other is decreased and, since these coils are connected in adjacent arms of the bridge circuit, the current responsive devices I4 and 15 will respond with high sensitivity to movement of the armature. With proper calibration either a direct indication or a graphic record may be obtained in terms of pressure variations.

In order to more fully appreciate the advantages to be obtained in the use of a pressure measuring device of the character described, as-

be readily'replaced with relatively small expense if corrosion occurs. It is further adaptableto be constructed for low cost in view of the fact that accurate machining operations necessary for no producing close flts or tolerances are not required.

to be. measured is at a high temperature, the

chambers and the like such, for example. as those produced in the oil chamber of a high capacity o-oil circuit breaker when the circuit is opened or sume for example that the highly flexible element 20 is omitted and that hot gases or liquids in the pressure chamber act directly on the main diaphragm or pressure sensitive element 30. In such a case, if the liquid or gas whose pressure in the combustion chambers of Diesel and other internal combustion engines. It will also be used to advantage for investigating steady-state or transient pressures existing in the cylinders of is steam locomotives. By the employment of pressure sensitive resilient members 30 in the form of diaphragms or beams of different stiffness and flexible elements 20 having active pressure surfaces 24 of different areas the pressure range may readily be changed to meet the desired conditions of measurement.

In accordance with the provisions of the patent statutes, I have described the principle of operation of my invention together with the apparatus which I now consider to represent the best embodiment thereof but I desire to have it understood that the apparatus shown is only illustrative and that the invention may be carried out by other means.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a device responsive to pressure changes of relatively hot fluids, a relatively stiff resilient member adapted to undergo elastic deformation due to applied forces, a flexible member in the form of a bellows having an active pressure surface and adapted to be mounted in sealed relation over an opening of a pressure chamber and in communication with the gas or liquid the pressure variations of which are to be measured, said bellows being relatively still in the transverse direction and having negligible stiffness as compared to said resilient member in an axial direction, and a push rod positioned beassess? tween said resilient member and said bellows for transmitting the forces applied to the active pressure surface of said bellows to said resilient member, said flexible bellows enclosing the cooperating end of said push rod so as to minimize the transfer of heat from the hot fluids to said push rod and said relatively stifi resilient member.

'. having negligible stiffness as compared to said I rod and said resilient member.

HANS P. KUEHNI. 

